1. Field of the Disclosure
This disclosure relates to power supplies and, more particularly, to control circuits for power supplies.
2. Background
LED lighting has become popular in the industry due to the many advantages that this technology provides. For example, LED lamps typically have a longer lifespan, pose fewer hazards, and provide increased visual appeal when compared to other lighting technologies, such as compact fluorescent lamp (CFL) or incandescent lighting technologies. The advantages provided by LED lighting have resulted in LEDs being incorporated into a variety of lighting technologies, televisions, monitors, and other applications.
It is often desirable to implement LED lamps with a dimming functionality to provide variable light output. One known technology that has been used for analog LED dimming is the phase angle dimming either by leading edge or trailing edge control. In a known example, a Triac circuit can be used that operates by delaying the beginning of each half-cycle of alternating current (ac) power, which is known as “phase control.” By delaying the beginning of each half-cycle, the amount of power delivered to the load (e.g., the lamp) is reduced, producing a dimming effect in the light output by the lamp. In most applications, the delay in the beginning of each half-cycle is not noticeable to the human eye because the variations in the phase controlled line voltage and the variations in power delivered to the lamp occur so quickly. For example, Triac dimming circuits work especially well when used to dim incandescent light bulbs since the variations in phase angle with altered ac line voltages are immaterial to these types of bulbs. However, flicker may be noticed when Triac circuits are used for dimming LED lamps.
Flickering in LED lamps can occur because these devices are typically driven by LED drivers having regulated power supplies that provide regulated current and voltage to the LED lamps from ac power lines. Unless the regulated power supplies that drive the LED lamps are designed to recognize and respond to the voltage signals from Triac dimming circuits in a desirable way, the Triac dimming circuits are likely to produce non-ideal results, such as limited dimming range, flickering, shimmering, blinking, and/or color shifting in the LED lamps.
Some causes of these non-ideal results in using Triac dimming circuits with LED lamps are in part due to the characteristic of the Triac itself. For example, a Triac is a semiconductor component that behaves as a controlled ac switch. Thus, the Triac behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal, causing the switch to close. However, many Triac dimming circuits include an inherent imbalance in phase angle and conduction angles from cycle to cycle. These variances in phase angle and conduction angles in consecutive cycles can result in voltage amplitude variations at the input and also as output current variations (i.e., ripple) at the output of the power converter. The output current ripple may appear as light fluctuations and shimmering in an LED load.
Corresponding reference characters may indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. Common but well-understood elements that are useful or necessary in commercially feasible embodiments are often not depicted in order to facilitate understanding of the various embodiments.